There is a general trend towards the miniaturization of a variety of battery-powered electronic devices. Extreme miniaturization of battery powered electronic devices forces designers to use a single miniature battery to achieve a reduction in the size of the entire device. Currently, hearing aids constitute a class of consumer electronic devices in which extreme miniaturization requires the use of a single miniature battery. However, there is a general trend to miniaturize many consumer electronic devices, such as portable stereo listening devices and pagers. Also, it is desirable to miniaturize many health-related electronic devices, such as on-patient medical monitoring devices which monitor heart rate or other life functions.
However, miniature batteries present numerous technical problems. The total stored energy in a battery decreases as the battery volume is decreased. The series resistance of the battery also increases as the battery size (diameter) is reduced. Also, miniature batteries typically contain only one electrolytic cell. A single electrolytic cell has a characteristic maximum source voltage. Typically, the voltage of a miniature battery during its useful lifetime is only slightly more than one volt depending upon the "freshness" of the battery.
The high series resistance and low source voltage of a miniature battery confronts circuit designers with difficult problems in designing efficient circuits which operate properly under these conditions. Commonly, complimentary metal oxide semiconductor (CMOS) integrated circuits, such as microprocessors, are designed to operate from a five volt power supply of constant polarity. Some high performance microprocessors are designed to operate from a three volt power supply. However, a miniature battery with a single electrolytic cell has a source voltage which is never substantially more than about 1.6 volts even when the battery is fresh. Consequently, many common CMOS circuits cannot be directly powered from a miniature battery. Generally, low voltage CMOS circuit design requires different design considerations compared with conventional CMOS circuit design.
Hearing aids are one example of an electronic device in which miniaturization of the total device size requires circuit designers to use a single miniature battery. A new generation of so-called "cosmetically-pleasing" miniature hearing aids is extremely popular. These miniature hearing aids include in-the-ear, in-the-canal, and completely in-the-canal hearing aids. Miniature hearing aids are largely or completely unobservable by others. Moreover, the placement of a miniature hearing aid within the ear canal provides several potential performance advantages, such as a reduction in distortion and noise; improved fidelity; and greater user comfort. Miniature hearing aids are one of the fastest growing areas of the hearing aid market.
Miniature hearing aid designers place a great emphasis in circuit design in allocating the use of limited space and battery power to efficient sound amplification. Many circuit designs which would be practical with a source voltage of 2 V to 3 V are not possible to achieve with a 1.1 V battery. For example, common diode protection circuits to guard against improper insertion of the battery are not feasible with miniature hearing aid batteries. Diode protection circuits are commonly used in many CMOS applications to protect against improper battery insertion. A diode typically has a 0.5-to-0.6 Volt turn-on voltage in the forward direction and is substantially non-conducting in the reverse direction up to a reverse-bias breakdown voltage (e.g., ten Volts). A diode protection circuit permits current flow when the battery is inserted with proper polarity but prevents current flow if the battery is incorrectly inserted. However, with a miniature hearing aid the 0.6 V voltage drop created by a diode protection circuit may render the rest of the CMOS circuit inoperable because the net operating voltage (e.g., 1.1 V-0.6 V=0.5 V) at best barely exceeds the threshold voltage of a single CMOS transistor. In the worst case, a diode protection circuit would cause such a severe voltage drop that the net operating voltage would be below the threshold voltage of a single transistor, making it impossible to design functional transistor circuits directly powered from the battery.
Low-voltage CMOS circuit design involves numerous tradeoffs related to the fact that the available supply voltage is too low to drive MOSFET transistors well into the saturation region. MOSFET transistor switches can only be driven into a high-conductivity ohmic regime if the effective supply voltage is significantly higher than the threshold voltage. As is well known, MOSFET transistors have several distinct operating regimes. There is a linear regime, corresponding to low drain-source voltages and/or low gate voltages. The drain current in the linear regime is commonly expressed by the mathematical equation: I.sub.d =2k[(V.sub.GS-V.sub.T)V.sub.DS -0.5 V.sub.DS.sup.2 ], where I.sub.d is the drain current, V.sub.GS is the gate source voltage, V.sub.T is the threshold voltage, V.sub.DS is the drain-source voltage, and k is a constant. In the saturation regime, generally corresponding to higher drain-source voltages and gate-source voltages, the drain current is expressed by the mathematical equation: I.sub.d =k[(V.sub.GS -V.sub.T).sup.2 ]. This saturation regime is also sometimes referred to as an ohmic region. Generally, it is difficult to drive a CMOS transistor into the ohmic region with a voltage source of less than about two volts. In particular, a gate-source voltage less than 1.1 volts is not consistent with a strong enhancement mode of operation because the quantity V.sub.GS -V.sub.T is too small.
Circuit designers designing CMOS circuits powered by 1.1 V hearing aid batteries must substantially redesign traditional CMOS circuits to minimize voltage drops between the battery and power amplification circuits. Low voltage CMOS transistors typically have a threshold voltage of approximately 0.4 to 0.6 volts. The drain-source voltage is preferably greater than 100 mV above the threshold voltage to achieve a strongly inverted channel. Consequently, it is difficult to operate more than two strongly inverted transistors in series with a 1.1 V battery because the available battery voltage (1.1 V) is just barely enough to bias two transistors to the strongly inverted channel regime (because 2.times.0.5 V=1.0 V). In some cases, there is insufficient voltage to drive even two transistors in series to the strongly inverted channel regime. Generally, for a low battery voltage it is difficult to achieve a large enough gate-source voltage to switch MOSFET transistors from fully-on to fully-off states.
Miniature hearing aids are also expensive, typically costing up to two-thousand dollars, which limits their widespread use. Part of the high cost of state-of-the-art miniature hearing aids is a result of the fact that the various components of typical behind-the-ear hearing aid circuits, such as input filters, analog amplifiers, digital electronic elements, filter circuits, and power supply protection circuits, cannot be straightforwardly integrated together because of fundamental incompatibilities (e.g., size limitations, voltage drops, and the problem of achieving a self-consistent CMOS design architecture). As a result, hybrid manufacturing processes are typically used to combine the function of different chips and discrete circuit elements together. Hybrid techniques, such as wirebonding or soldering different discrete components such as discrete capacitors, filters, amplifiers, and power supply protection circuits together requires labor intensive and space consuming procedures, such as mounting the components in close proximity to one another and wirebonding or soldering electrical connections between the components.
The low battery voltage of miniature batteries also makes it difficult to achieve a high level of sound amplification. The power of an amplified signal depends upon the square of the voltage of the amplified signal. Consequently, it is difficult to design powerful amplifiers if the available supply voltage is low, since this limits the maximum potential output voltage swing of the amplifier. Additionally, the maximum voltage swing may be limited by other effects, such as noise and distortion considerations. For example, linear audio amplifiers used in hearing aids typically have a poor power conversion efficiency if they are operated in a voltage regime along the load line of the amplifier where harmonic distortion is acceptably low.
Class D amplifiers have many potential performance advantages compared with linear amplifiers, such as the potential for higher power conversion efficiency and higher power output. Class D operation is typically achieved by configuring four transistors to operate in a voltage switch-mode of operation in which diagonal pairs of transistors are alternately switched on and off. Class D amplifiers typically use complementary transistor pairs consisting of four switches, two of which have the equivalent of inverted inputs by virtue of a change in transistor type.
In Class D amplifier technology, a linear signal is first converted into a pulse width modulated (PWM) signal so that the input signal strength is proportional to the width of the pulse. Preferably, the PWM input signal is amplified across a range corresponding to both the positive and negative battery potential (so-called "rail-to-rail" amplification). For example, the PWM signal may be split to drive two inverter chains. The PWM signal and inverted PWM signal may be used as inputs to an H-bridge amplifier comprised of complementary n-channel and p-channel transistors. When the PWM signal is "high" one set of complementary transistors switches on such that the output voltage is raised to the positive rail voltage. When the PWM signal is "low," the other set of complementary transistors is switched on such that the output voltage is switched to the negative rail voltage. In theory, this rail-to-rail mode of amplification may increase the maximum power output of a hearing aid by a factor of four, since the amplifier is operated across a voltage range corresponding to twice the battery voltage (i.e., .+-.V.sub.b, where V.sub.b is the battery voltage). The amplified PWM signal may then be converted back into a linear signal using integrator techniques (typically by using the hearing aid receiver coil as the integrator to smooth out the high frequency PWM signal components).
Unfortunately, it is difficult to effectively use Class D amplifier circuits in integrated hearing aid circuits. Efficient "rail-to-rail" amplification using Class D amplifiers requires that the transistors switches have a low on-resistance. In theory, a power conversion efficiency in excess of 90% is possible if the effective on-resistance of the switches is sufficiently low. However, the nominal battery voltage (1.1 V) is only slightly above typical n-channel and p-channel MOSFET turn-on voltages (e.g., about 0.5 V). As is well known, MOSFET circuits which are only weakly driven above their threshold voltage have a substantial resistance per unit of gate width. Consequently, there is an important tradeoff between the size of a H-bridge amplifier and its power conversion efficiency. Typically, it is difficult to simultaneously achieve both a compact size and a high power conversion efficiency for a H-bridge amplifier powered by a low-voltage battery.
The low efficiency of sound amplification and small battery size limit the lifetime of hearing aid batteries. Hearing aid batteries typically have a useful lifetime in excess of one-hundred hours. The short battery lifetime is partially a consequence of the small battery size alone, which reduces the maximum stored energy and increases the battery resistance. The problem of short battery-lifetime is further compounded by the relatively low efficiency of sound amplification that can be achieved with conventional linear amplifiers because of the low source voltage (typically less than 1.1 Volts) and high battery resistance of miniature batteries.
The frequent battery replacement in miniature hearing aids is both costly and inconvenient to the user. Frequent battery replacement can also be frustrating to users because the batteries used in miniature hearing aids are so small that it is easy for users to insert the battery with the wrong polarity. This is a problem because a significant fraction of the users of such hearing aids have poor close-up vision, poor hand coordination, or poor finger dexterity. Even if the electronics are not damaged by incorrectly inserting the battery, the user still has the frustration of having to frequently re-insert a small battery to attain the correct polarity. Moreover, frequent re-insertion of the battery increases the likelihood that portions of the battery case, such as plastic lip-regions, may be damaged.
Miniature hearing aids would thus be more convenient to use if a compact battery polarity correction circuit could be integrated with other electronics so that the hearing aid would function properly regardless of which direction the battery was inserted. Additionally, a battery polarity correction circuit could potentially increase battery lifetime. The size of miniature hearing aid batteries is so small that the battery must be shaped and/or imprinted to aid users in visually distinguishing the polarity of the battery. Common methods, such as shaping one end of a battery and imprinting "plus" and "minus" signs on the battery terminals to facilitate correct battery insertion use upwards of 20% of the available battery volume and reduce battery capacity compared to a battery shaped as a true cylinder with the same maximum diameter and maximum height. Also, a significant portion of the cost of manufacturing miniature batteries is associated with the cost of machining the shaped end. A low resistance battery polarity correction circuit would permit less expensive, blank-faced batteries with a significantly longer lifetime to be used.
An efficient miniature hearing aid with a battery polarity correction function would make battery replacement both a less burdensome and more infrequent chore for the hearing aid user. A highly desirable miniature hearing aid would integrate together battery polarity adjustment circuits, efficient Class D amplifiers, and other valuable electronic functions into one compact circuit in order to permit longer battery life, increased functionality, lower manufacturing costs, and to make the hearing aid more convenient for the user.
Unfortunately, the small size of miniature hearing aids combined with the low voltage of the battery makes it difficult to integrate together a voltage polarity correction function, an efficient amplifier, and other hearing aid electronics into a miniature hearing aid. It is desirable that the hearing aid amplifier and receiver is designed to fit into the ear canal in order to achieve a reasonable acoustical coupling of sound emitted from the hearing aid into the cochlea. Although there are individual variations in size and shape, the human ear canal is generally S-shaped and has a first bend region and a second bend region. The cross-sectional diameter of the human ear canal also varies along its length. The industry standard is that the electronics package designed to fit close to the ear drum (i.e., about 0.100" from the ear drum) should be a rectangular package no larger than about 0.090" thick, 0.110" wide, and 0.200" long. This space constraint makes it difficult to achieve circuits in which the individual transistors are wide enough to have a low resistance when driven by a 1.1 V battery. As is well known by those skilled in the art, CMOS transistors which are biased only slightly above threshold have a non-negligible resistance per unit of gate width. Consequently, the transistors in Class D amplifiers and in polarity corrected voltage sources must be made extremely wide in order to achieve a reasonable series resistance for the integrated circuit and to maintain a sufficiently high operating voltage. Although the circuit area of previously known H-bridge hearing aid amplifiers varies somewhat, U.S. Pat. No. 4,592,087, describes CMOS Class D amplifiers driven off of 1.2-1.5 V batteries which have a circuit area of 0.075".times.0.090" (with a corresponding thickness of 0.009"). Previously known battery polarity correction circuits commonly utilize a four transistor bridge-type circuit. However, in order to achieve a low series resistance of the polarity correction circuit, the transistors are typically quite large. For example, commercially available battery polarity correction circuits typically have a circuit area of 0.140".times.0.100" (with a die thickness of 0.010") in order to achieve a series resistance of 3.5 ohms.
Another obstacle to directly integrating a voltage polarity correction function with a Class D amplifier is the potential noise problems created by directly integrating power supply correction circuits that drive all of the hearing aid electronics. For example, if Class D amplifiers are used to drive the speaker of a miniature hearing aid, the amplifier section will consume large pulsed currents compared to other electronic components, such as small signal linear pre-amplifiers. The large difference in the load current of the Class D amplifier compared to other sections driven off of the common battery will produce spike noise, which can severely degrade the performance of all preceding amplifiers or require sophisticated circuit techniques to manage this effect. Additionally, there typically are other parasitic currents in the amplifier structure which further exacerbate the problem of spike noise. It is difficult, given the limited available room, to incorporate sufficiently large integrated or discrete capacitors, such as 5-10 .mu.F capacitors, to effectively filter the small signal loads from the spike noise. Additionally, the series resistance of a battery polarity correction circuit contributes to the problem of spike noise. As described above, even comparatively large polarity correction circuits have a series resistance of 3.5 ohms. This is comparable to the resistance of a miniature battery. The effective series resistance of the equivalent voltage source is at least doubled, which causes a corresponding increase in spike noise if a Class D amplifier is driven off of the polarity corrected voltage source.
Still another obstacle to directly integrating a voltage polarity correction function and a Class D amplifier is caused by substrate biasing problems. CMOS transistor circuits require that a correct polarity be maintained not just for the source and drain contacts of each transistor but also for the entire substrate contact. CMOS battery polarity correction circuits have the two battery terminals as circuit inputs and also have one battery terminal connected to the substrate. In response, the battery polarity correction circuit produces a "high" terminal" output and a "low" terminal output whose polarity is constant regardless of which direction the battery is inserted into the hearing aid. As is disclosed in U.S. Pat. No. 5,661,420, the two outputs of a battery polarity correction circuit on a first chip can be used to drive amplifier sections on a second chip by mounting the polarity correction circuit such that its two outputs form the inputs to subsequent amplifiers on another substrate. However, direct integration of such a battery polarity correction circuit with a CMOS-based amplifier on the same chip is frustrated by the fact that the substrate voltage is still determined by the battery polarity because the circuits share a common substrate. Even if the high and low input voltages to the amplifier from the battery polarity correction circuit remain constant regardless of battery polarity, the CMOS circuits of many amplifiers would not function properly.
As is well known in the art of CMOS circuit design, a change in the polarity of the substrate bias may shift the threshold voltage of CMOS transistors, which is an especially serious problem in low-voltage applications where key transistors may be only slightly driven above threshold. A commonly used first-order approximation is that the threshold voltage shifts according to the square root of the substrate-source bias, V.sub.BS. However, for the case where V.sub.BS is comparatively low, a more accurate analysis is required using equations well known to those of ordinary skill in the art. The threshold voltage will depend upon the MOSFET doping and also upon the source-to-drain distance. Calculations by the inventors indicate that for common MOSFET doping choices that a 0.9-to-1.6 V substrate bias produces a shift in threshold of about 0.13-to-0.15 V. This shift is significant if the MOSFET transistors are barely biased to the ohmic regime. However, even if the transistors are biased into the ohmic regime, a significant change in transistor current and on-resistance may still occur. The drain-source resistance of a MOSFET transistor with a low drain-source voltage may be approximated as being inversely proportional to V.sub.GS -V.sub.T. If the gate source voltage is low (e.g., less than about 1.1 Volts), then with common threshold voltages of about 0.5 Volts, a change in substrate bias polarity can produce over a 25% change in on-resistance of a MOSFET.
It is desirable that a miniature hearing aid integrated circuit integrates a compact, efficient Class D amplifier, a battery polarity correction function, and other valuable electronics into a circuit suitable for miniature hearing aid applications. However, there is an inherent contradiction to directly integrating a voltage polarity correction function with a Class D amplifier suitable for miniature hearing aid applications using previously known design approaches. The low-voltage power supply imposes one set of important design constraints. Previous CMOS low-voltage design approaches utilize comparatively large transistors to reduce resistive voltage drops. However, circuit size is another important constraint. The total circuit should be no larger than the industry standard for in-the-ear canal designs and is preferably substantially smaller to reduce cost and permit other valuable electronic functions to be incorporated on the same chip. Additionally, the problem of successfully integrating a compact voltage polarity correction function with other circuits on the same chip has not been addressed in previously known design approaches.
Many of the same issues which are currently important for miniature hearing aids are relevant to other consumer electronic devices. For example, many pagers have transducers which make an audible noise or vibrate. As pagers continue to be miniaturized, eventually they will comprise a miniature battery, a low-frequency transducer, and CMOS control circuits. Portable stereos may soon consist of hearing-aid sized units designed to worn in the ear. Generally, there is a wide variety of electronic devices for which further miniaturization will require that a single miniature battery be employed. The same general problem of designing a compact, efficient, high power amplifier circuit driven by a single low voltage battery is common to a variety of these applications. Also, a variety of consumer electronic devices would benefit if miniature blank-faced batteries, which offer important advantages in terms of user convenience, battery cost, and battery lifetime, could be utilized.
What is desired is a new integrated circuit design approach which permits an efficient Class D amplifier with battery polarity insensitivity to be achieved in a compact circuit powered by a miniature 1 Volt battery.